For purposes of discussion, a wood-working moulding machine may be taken as an example. Such machines are per se well known as comprising in one form an elongated base having work supporting and guiding surfaces along which an elongated board or wooden strip is moved lengthwise by appropriate adjustable rate feed drive means. Spaced along the work feed path are one or more power-driven spindles, each of which in many or most cases carries a plurality of cutting elements or knives spaced circumferentially around a head attached to the spindle. The cutters may be integrally formed on the head or more frequently take the form of replaceable inserts or blades. Typically, there are four spindles--two lying on horizontal axes so that their cutters shape and/or smooth off the upper and lower surfaces of the passing wood workpiece, and two lying on vertical axes such that their cutters similarly act on the left and right edge surfaces of the workpiece.
It has been known for years that the plural cutters on a given spindle in such a machine all participate to some degree in the removal of material from the workpiece, but that the final surface finish may appear as if only the "high" or largest radius one of the cutters took the last bite. The final surface finish on an elongated moulded wood strip exhibits "knife marks" which lie transverse to the direction of feed; and an index of the quality of the surface finish is designated as higher if a greater number of shallow knife marks exists per inch of workpiece length. The knife marks per inch (and thus surface quality) on a finished piece are (a) proportional to the number of cutters spaced around the spindle head and the rotational speed (r.p.m.) of the spindle, and (b) inversely proportional to the feed rate (usually expressed in feet per minute) of the workpiece--providing that all cutters have their edges lying on equal radii, or more precisely that all cutting edges move around the same imaginary surface of revolution concentric with the spindle axis. When that latter provision is met, then all cutters participate in creating the finished surface; otherwise, one cutter is the highest and the knife marks per inch turn out to be as if only the high cutter produced the final workpiece surface.
Almost inevitably (unless in situ "jointing" is performed, as described below) one of the plural cutting edges associated with a given spindle will to some degree have a larger effective radius than the others (i.e., will move along a surface of revolution radially larger than the others). Even if cutters on a given spindle head are carefully sharpened and trued on a grinder in a tool room to have what seemingly are equal effective radii, when that spindle head is then placed in the machine tool, one cutter will almost immediately show up as the "high tooth" and create a low per inch knife mark count on finished workpieces. It is believed that this occurs because of differential thermal expansion or slight dimensional differences between a tool room grinder and the machine tool itself. Beyond that, however, even when a plural knife spindle initially produces the expected high quality surface finish, wear on the knife edges will be unequal over a relatively short time period, and one cutter edge inevitably becomes "higher" than the others so that surface finish quality degrades. In either case if a given work surface quality is to be maintained, then the feed rate must be reduced with consequent reduction in productivity and increase in production costs. Increasing spindle r.p.m. would have the same compensating effect as decreasing feed rate, but there are limits imposed on r.p.m. by safety and machine tool design factors.
In the wood moulder machine art, the "high tooth" problem has been dealt with by a procedure known as "jointing". The origin of that term is unknown; it is a confusing misnomer not to be confused with the wood working procedure known as "joining" practiced by joining machines. In any event, "jointing" as applied to moulding machines involves bringing an abrasive stone progressively inwardly toward the cutting edges of a rotating plural-cutter spindle so as to wear off, by grinding action, the higher (larger radius) cutting edges until all cutting edges move at the same effective radius, i.e., describe the same surface of revolution about the spindle axis. This is done at periodic intervals (e.g., every 4 to 8 hrs.) depending on the rapidity with which the surface finish on produced workpieces degrades. It is carried out in situ in the machine tool which contains the plural cutter spindle and it permits the finished surfaces to be kept within a chosen, acceptable range of quality (knife marks per inch) without reducing feed rates and thus workpiece production rates.